Small-angle neutron scattering of precipitates in Ni–Ti shape memory alloys

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

“…In a related study, Somsen [37] obtained diffraction patterns nearly identical to the one inset in Figure 4(b), and the volume fraction and size of Ti 3 Ni 4 precipitates was estimated to be below 0.3 vol pct and 1 nm, respectively. [36] The as-received cold-drawn material contained a high density of dislocations and martensite plates, resulting in a heavy strain contrast, as evident in Figure 4(c). The size of precipitates was not confirmed in the as-received cold-drawn material; however, it is reasonable to assume that the precipitates did not grow from their size in Figure 4(b) during the cold drawing.…”

“…The selected area diffraction pattern shown in the inset was obtained after tilting the specimen to the Ͻ111Ͼ zone axis. The right inset shows an indexed schematic of the diffraction pattern with diffuse intensity resulting from 1/3 Ͻ110Ͼ reflections [36] (arrowed). Kompatscher et al [36] studied Ni-rich NiTi alloys and employed small-angle neutron scattering to examine the volume fraction and size of Ti 3 Ni 4 precipitates in near solutionized states.…”

“…The right inset shows an indexed schematic of the diffraction pattern with diffuse intensity resulting from 1/3 Ͻ110Ͼ reflections [36] (arrowed). Kompatscher et al [36] studied Ni-rich NiTi alloys and employed small-angle neutron scattering to examine the volume fraction and size of Ti 3 Ni 4 precipitates in near solutionized states. In a related study, Somsen [37] obtained diffraction patterns nearly identical to the one inset in Figure 4(b), and the volume fraction and size of Ti 3 Ni 4 precipitates was estimated to be below 0.3 vol pct and 1 nm, respectively.…”

“…Note that the small precipitates formed after the present heat treatment cause only weak intensities even in overexposed diffraction patterns. For a recent detailed electron microscopy study, the reader is referred to Kompatscher et al [36] and Somsen. [37] For the scope of the present article, it is sufficient to note that even substantially higher annealing temperatures than those used in the present study will result in coherent precipitates, [31,32] and precipitates will not lose coherency fully before they reach a size of approximately 150 to 300 nm, depending on the degree of cold work.…”

On the effect of room temperature ageing of Ni-rich Ni–Ti alloys

“…Ageing temperatures around 470-570 K are considered as low [1][2][3][4][5][6][7][8][9][10][11][12][13]: this temperature range is well below the recrystallization temperature close to 870 K, and it is believed that no microstructural changes like precipitation occur below approximately 470 K. For instance, Kim and Miyazaki did not observe by transmission electron microscopy any traces of precipitation after ageing for 3.8 ks at 370 K of a Ni 50. 9 Ti 49.1 alloy quenched from 1070 K [1]. As for the origin of the effect of lowtemperature ageing it is generally agreed that it stems from the formation of precipitates and related variation of the Ni content of the matrix.…”